Southern Ocean deep-water carbon export enhanced by natural iron fertilization

Iron fertilization, naturally The ocean's importance in storing carbon is widely recognized, as is the importance of iron as a limiting nutrient in much of the global ocean. But quantifying the increase in long-term carbon storage in response to the lifting of iron limitation has proved difficult. The CROZEX experiment, a cruise on-board the RRS Discovery , set out to test the hypothesis that the observed north–south gradient in phytoplankton concentrations near the Crozet Islands in the Southern Ocean is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. The data support the hypothesis: carbon export fluxes to the deep from the fertile waters were two to three times greater than those fluxes from an adjacent high-nutrient low-chlorophyll area not fertilized by iron. The efficiency of carbon export was somewhat greater than that reported in experiments where iron is added artificially, a possible consequence of large losses of the artificially added iron, but smaller compared to that reported from a naturally induced bloom, possibly related to the importance of horizontal iron supply. It is found that carbon export fluxes to the deep ocean from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon export fluxes from an adjacent high-nutrient low-chlorophyll area not fertilized by iron. These findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon 1 , 2 , 3 . Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified 3 . Here we report data from the CROZEX experiment 4 in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times